Sound-absorbing system



Nov. 8, 1960 Filed Dec. 15, 1955 FIG. 7

FIG. 2

R. MULLER ETAL 2,959,242

SOUND-ABSORBING SYSTEM 4 Sheets-Sheet 1 JNVENTOR.

Nov. 8, 1960 R. MULLER EFAL 2,959,242

SOUND-ABSORBING SYSTEM Filed Dec. 15, 1955 4 Sheets-Sheet 2 J. Sabine 100 IN VEN TOR.

RUDOLF MULLER. ULRICH FRITZE, HANS -W/LLI PAFFRATH.

3, 1960 R. MULLER ETAL 2,959,242

SOUND-ABSORBING svsmu Filad Dec. 15, 1955 4 Sheets-Shoot a at Sabine 0 l n l 1 l 1 1 l l I l g F IG. 5

INVENTORS.

1 RUDOLF MIILER, ULR/G-IFRITZE, HA/vs-w/Lu PAFFRATH Nov. 8, 1960 R. MULLER E'AL 2,959,242

SOUND-ABSORBING SYSTEM Filed Dec. 15, 1955 4 Sheets-Sheet 4 7o cl Sabine INVENTORS. RUDOLF Ml'lLER. ULRICH FRITZE. HANS-WILL! PAFFRATH.

United States Patent SOUND-ABSORBING SYSTEM Rudolf Miiller, Leverkusen, Ulrich Fritze, Koln-Stammheim, and Hans-Willi Patfrath, Koln-Deutz, Germany, assignors, by direct and mesne assignments, of one-half to Farbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, a corporation of Germany, and onehalf to Mobay Chemical Company, Pittsburgh, Pa., a corporation of Delaware Filed Dec. 15, 1955, Ser. No. 553,350

Claims priority, application Germany Dec. 21, 1954 2 Claims. (Cl. 181-33) This invention relates to sound-absorbing systems comprising a mat of elastic foam covered with a thin layer of a normally air-impermeable material, said layer containing fine perforations.

It is known to produce sound-absorbing systems from porous or fibrous materials. The sound-absorbing effect of such systems is caused by the conversion of the airtransmitted sound energy into heat as a result of friction on the walls of the cavities contained in said porous or fibrous materials. Sound-absorbing systems of this type absorb predominantly high frequencies.

Another category of sound-absorbing systems is made up of elastic material which is excited by the sound waves to produce mechanical oscillations and dissipates sound energy owing to its high internal damping. These systems (resonators) absorb predominantly low frequencies.

A further group of sound-absorbing systems is based on the principle of the so-called cavity resonators (Helmholtz resonators). They are usually constructed by mounting a lath structure on the wall of the room, in which the volume of sound is to be reduced, and sealing off the structure toward the room by means of perforated plates. These sound-absorbing systems are particularly suitable for absorbing low frequencies.

Since the known sound-absorbing systems are rather selective with respect to the frequencies they absorb, it has heretofore been necessary to use a combination of these systems in order to achieve a sound-absorption over a wide range of frequencies. It has already been proposed to produce such combinations by mounting perforated plates on laths disposed at a certain distance from the wall and by filling the space between the wall and said perforated plates with sound-absorbing fibrous materials. However, the assembly of such systems is costly and takes a considerable amount of time since it involves three mechanical operations, i.e., mounting the spacing laths on the walls, attaching the perforated plates to said spacing laths and filling the space between the plates and the wall with a fibrous material.

It is an object of the present invention to provide soundabsorbing systems which absorb sound waves over a wide range of frequencies. Another object is to provide soundabsorbing systems which are inexpensive and can easily be constructed. Further objects will appear hereinafter. Figure 1 is a diagrammatic cross section of a wall with a piece of polyurethane foam bonded thereto.

Figure 2 is a diagrammatic cross section of a wall with a piece of polyurethane foam bonded thereto and having a covering layer of lacquered paper thereon.

Figure 3 is a diagrammatic cross section of a wall with a piece of polyurethane foam bonded thereto having a covering layer of perforated lacquered paper thereon.

Figures 4 through 6 are line graphs showing the per 2,959,242 Patented Nov. 8, 1960 cent sound-absorption in Sabine units for the indicated frequencies in cycles per second.

It has now been found that a mat of elastic foam covered with a thin layer of a normally air-impermeable material, said layer containing fine perforations, constitutes a very effective sound-absorbing system which is capable of absorbing lowand high-frequency sound waves.

Mats of elastic foam suitable for use in the practice of the invention can readily be obtained by cutting blocks of polyurethane, polyvinyl, rubber or other elastic plastic foam. The foam, the pore size of which may well amount to from 0.3 to 3 mm., should have a rebound within the range of 2035%. The surface layer of airimpermeable material can be produced in any desired manner. Thus, plastic foils or paper can be attached to the mats of elastic foam by welding or gluing. If desired, the surface layer can be painted or lacquered before or after being attached to the mats of elastic foam. The surface layers can also be produced by direct application of lacquer solutions or film formers in paste form. The perforations in the surface layer can be made by any desired mechanical means, for example with the aid of a pin roller. The thickness of the mats and the size and number of the perforations in the surface layer may be varied widely. Very satisfactory results are obtained if the mats are about 0.5 to 3 inches thick and the perforations are about 1 to 10 mm. apart and about 0.1 to 3 mm. in diameter.

A preferred embodiment of the sound-absorbing systems of the present invention is obtained by gluing mats of elastic polyurethane foam to the walls and/or the ceiling of the room in which sound is to be absorbed, gluing sheets of paper to said mats, coating the paper layer with a conventional paint, lacquer or varnish, and subsequently forming fine perforations in the coated paper layer.

The sound-absorbing characteristics of the systems of the invention are caused by two effects: The surface layer acts as a sound-absorbing resonator for low frequencies in conjunction with the damping by the elastic material, while the sound waves of higher frequency penetrate through the perforations of the skin into the pores of the elastic material and are converted therein into heat.

In addition to the very desirable feature of absorbing over a wide range of frequencies, the sound-absorbing systems of the invention have the further advantage that they can be arranged without joints on the walls of the inclosure and give the impression of a completely uniform surface, since the fine perforations are not visible from a distance.

Although the sound-absorbing systems of the invention are particularly suitable for use in rooms, they may also be used in other inclosures where noise is to be suppressed, such as cabins of airplanes and ships, automobiles and ducts of air-heating systems.

The following example further illustrates the invention without in any way limiting it.

Example The example shows by reference to Figures 1-6 the arrangement of the separate sound-absorbing elements and the dependence of the frequency response and of the degree of absorption on the weight per unit area of the surfacing layer and upon the number of perforations.

According to Figure 1, a mat 2 of elastic polyurethane foam is glued to the wall surface 1, said mat having a thickness of 3 cm. The absorbing effect of this system is shown by a curve a in Figure 4.

The mat is cut from a foamed block material which may be foamed in well-known manner for instance by mixing the following ingredients:

100 parts by weight of a polyester prepared by thermal condensation of 15 inols of adipic acid, 16 'mols of diethylene glycol and 1 mol of trimethylol propane; OH number 60, acid number below 1,

49 parts by weight of toluylene diisocyanate,

11 parts by weight of the following mixture:

3 parts by weight of bis-(diethylaminoethanol)-adipate,

1 part by weight of diethylamine oleate,

0.3 part by weight of glycerol, I I p 1.5 parts by weight of a sulfonated ricinolic acid (water content 54%), I V

1.5 parts by weight of the sodium salt of a sulfonated casto-r oil (water content 54%),

2 parts by Weight of water.

vAccording to Figure 2, paper 3 is glued to the surface of the system according to Figure 1 and one or two coatings of lacquer (oil paint) 4 are applied to said paper. As paper waste sheets are taken with a weight per unit area of 60 g./m. One or more of these sheets are glued onto the system depending on the desired degree of absorption. The degree of sound absorption of this system is shown by the curve b of Figure 4. The system now acts exclusively as a resonator.

If the final layer is now provided with perforations 5 with a diameter of 0.5 mm. and a spacing of 5 mm.,

these perforations being formed for example by means of a pin roller, there is obtained the system shown in Figure 3, the sound-absorbing effect of which is represented by the curve in Figure 4. This curve 0 showing an absorption over a wide range of frequencies is ,produced by the resonating action of the system according to curve b and the absorption of the porous material according to curve a.

Figure shows the degree of absorption as a function of the weight per unit of area of the surfacing layer, more especially the shifting of the maxima to higher frequencies as the weight per unit of area decreases. Curve d is measured with a surfacing layer having a weight of 2.1 g./cm. curve e is measured with a surfacing layer having a weight of 4.23 g./cm. and curve 1 is measured with a surfacing layer having a weight of 6.19 g./cm.

Finally, Figure 6 shows the change in the degree of absorption as a function of the number of the perforations, the degree of absorption rising with an increasing I number of perforations. Curve g is recorded with an imperforate surfacing layer, curve It with a perforation of 5 holes per cm? and curve i with a perforation of 10 holes per cm. (diameter ofjperforations 0.5 mm.).

The above description and example are intended to be illustrative only. Any modification of or variation therefrom which conforms to the spirit of the invention is intended to be included within the scope of the claims.

What is claimed is:

1. An improved sound-absorbing system adapted for use in rooms and the interior of airplanes, ships, automobiles, ducts of air-heating systems and the like which comprises a layer of polyurethane foam having a porous surface and having a pore size of from about 0.3 mm. to about 3 mm., the rebound of said foam being from about 20 percent to about 35 percent, said layer of polyurethane foam being provided with an adherent covering layer of normally air-impermeable material having perforations therein of from about 0.1 to about 3 mm. diameter spaced from about 1 to about 10 mm. apart.

'2. The improvement in polyurethane foam soundabsor-bing systems adapted for use in rooms and the interior of airplanes, ships, automobiles, ducts of airheating systems and the like which comprises a layer of polyurethane foam having a porous surface and having a pore size of from about 0.3 mm. to about 3 mm. the rebound of said foam being from about 20 percent to about 35 percent, said polyurethane foam having an adherent covering layer of normally air-impermeable material having perforations therein of from about 0.1 to about 3 mm. diameter spaced from about 1 to about 10 mm. apart.

References Cited in the file of this atent UNITED STATES PATENTS 2,018,207 Giainbertoni Oct. 22, 1935 2,297,218 Heinrich et al. Sept. 29', 1942 2,562,711 Gessler et al. July 31, 1951 2,753,276 Brochhagen et al. July 3, 1956 2,903,380 Hoppe et al. Sept. 8, 1959 V I FOREIGN PATENTS 510,707 Great Britain Aug. 4, 1939 516,512 Belgium Jan. 15, 1953 OTHER REFERENCES V Fubli c'ation; Plastics Institute Transactions, 1948, article by A. Cooper, beginning on page 51, pages and 67 are pertinent. 

1. AN IMPROVED SOUND-ABSORBING SYSTEM ADAPTED FOR USE IN ROOMS AND THE INTERIOR OF AIRPLANES, SHIPS, AUTOMOBILES, DUCTS OF AIR-HEATING SYSTEM AND THE LIKE WHICH COMPRISES A LAYER OF POLYRETHANE FOAM HAVING A POROUS SURFACE AND HAVING A PORE SIZE OF FROM ABOUT 0.3 MM. TO ABOUT 3 MM., THE REBOUND OF SAID FOAM BEING FROM ABOUT 20 PERCENT TO ABOUT 35 PERCENT, SAID LAYER OF POLYURETHANE FOAM BEING PROVIDED WITH AN ADHERENT COVERING LAYER OF NORMALLY AIR-IMPERMEABLE MATERIAL HAVING PERFORATIONS THEREIN OF FROM ABOUT 0.1 TO ABOUT 3 MM. DIAMETER SPACED FROM ABOUT 1 TO ABOUT 10 MM. APART. 